Igloo Basement: Polished andesite under red carpet instead of chiseled stone bricks

The floor of the igloo's basement has a clear pattern of chiseled stone bricks and normal stone bricks.

Why is the corner block under the carpet polished andesite? The whole room has stone bricks in some form, may it be mossy, cracked, chiseled or normal. Why is there andesite hiding under the carpet?